1. Field of the Invention
This invention relates to a resin-bonded magnet consisting mainly of iron and having an improved level of rustproofness, and a process for making the same. More particularly, it is concerned with a resin-bonded rare earth-iron-boron (hereinafter referred to as Nd-Fe-B) magnet and a process for making the same.
2. Description of the Prior Art
It has long been known that there are alloys or compounds consisting mainly of iron (i.e., containing at least 50 atom % of iron), and having very high magnetic properties, since iron is an element having a higher saturation magnetic flux density at room temperature than any other element does, and that those alloys or compounds can be used for making resin-bonded magnets having very high magnetic properties. Specific examples of those alloys or compounds are Nd.sub.2 Fe.sub.14 B, SmFe.sub.12 and Fe.sub.16 N.sub.2 which have all been recently developed. All of these alloys or compounds have, however, the drawback of being easily oxidized and getting rusty, as they contain a high proportion of iron. This is particularly the case with Nd-Fe-B magnets for which there has recently been an increasing demand. They easily get rusted in a highly humid environment. Various measures have, therefore, been proposed for making those magnets rustproof, and include the coating of the surface of a resin-bonded Nd-Fe-B magnet with an acrylic or epoxy resin (Japanese Patent Applications Laid-Open Nos. 244710/1988 and 244711/1988), and the coating thereof with a fluorine-containing resin (Japanese Patent Application Laid-Open No. 168221/1986). They also include electro-deposition on the magnet surface, and the plating of the magnet surface with nickel, or other metal.
All of these methods are, however, unsatisfactory from the standpoint of economy or corrosion resistance, or both. The coating of the magnet surface with a resin can be done at a low cost, but is incomplete as a rustproofing method, since it is difficult for any resin to shut off oxygen and water completely. Electrodeposition is costly. Metal plating is also costly, and moreover involves every likelihood that even a small amount of coating solution remaining on the magnet surface may cause corrosion to propagate over an enlarged area.
The use of a reducing resin has also been proposed. For example, Japanese Patent Application Laid-Open No. 290209/1989 discloses a rare earth alloy magnet coated with a film containing an alkylphenol, or alkyl-polyhydric phenol resin. The resin is, however, used only for coating the surface of a sintered magnet, and cannot always be said to be effective as a bonding resin for a bonded magnet.
We, the inventors of this invention, found that a special resin which was obtained by polycondensation reaction from tannic acid, phenol and formaldehyde (hereinafter referred to as a "polytannin resin") was very effective for rustproofing a bonded Nd-Fe-B magnet. As a result of further research, however, we have found that bonded magnets made by using this resin as a binder are not always satisfactory in strength, apparently because it is a resin intended primarily for coating a surface. This is particularly the case with a cylindrical magnet having a small wall thickness and a large bore diameter. It cracks or chips during handling, and hardly can withstand any practical use. Thus, we have reached the conclusion that this resin is not suitable as a binder, despite its excellent rustproofing power.
The known methods for improving the corrosion resistance of a bonded magnet include not only the use of a rustproofing synthetic resin as a binder, but also the coating of a magnetic powder, or the surface of a magnet with a rustproofing synthetic resin. A combination of these methods may be employed for achieving a still higher level of rustproofness. However, insofar as none of the resins as hereinabove mentioned is suitable as a binder, but all of them are used only for coating a magnet to render it rustproof, it has been necessary to employ another resin as a binder. This has been an obstacle to the realization of a simplified manufacturing process.